SCT Corrector Plate Questions

[rocco (Richard)]

I’ve been looking around the net regarding how SCT correcting plates are made.
It appears most commercially made plates are made with the ‘wave’ correction on one surface and flat the other side.
Lots of diagrams of SCTs show the corrector plate mounted in the telescope tube with the ‘wave’ surface facing towards the main mirror. Other diagrams show the plate with the ‘wave’ surface facing outwards in the tube and the flat side towards the main mirror.
So which way is correct? Or doesn't it matter? On my LX200, the corrector plate appears to be flat when I look at the front of the scope, so presumably the 'wave' surface is inside? (I am way too chicken to ever want to pull the corrector plate off to check this!)
How thick is the plate, or does this thickness depend on the aperture of the scope - so would a 14” plate be thicker than an 8” plate?
And apparently it is perfectly permissible to have a plate that is corrected on both sides and some diagrams reflect this, too.

[GeoffW1 (Geoff)]

Hi,

You would not remove the plate just to clarify this question, but be assured that it is not a big deal if the need does come, at least for Celestrons.

There is a mile of information available, and provided you observe a few simple precautions, all should go well. It has for me.

Cheers

[Barrykgerdes]

The reason for the corrector plate is to alter the light path towards the edge of the mirror (spherical) in a progressive manner to correct for spherical aberation. To do this truly accurately the mirror the corrector and the secondary should be at a specific position relative to each other. However the SCT's get their primary focus by moving the main mirror and the differences are minute. It won't make any practical difference which way the corrector surface faces as long as it does the job.

That's my understanding of it.

Barry

[multiweb (Marc)]

Originally Posted by rocco

I’ve been looking around the net regarding how SCT correcting plates are made.
It appears most commercially made plates are made with the ‘wave’ correction on one surface and flat the other side.

That's correct.

Originally Posted by rocco

Lots of diagrams of SCTs show the corrector plate mounted in the telescope tube with the ‘wave’ surface facing towards the main mirror. Other diagrams show the plate with the ‘wave’ surface facing outwards in the tube and the flat side towards the main mirror.
So which way is correct?

Correct again. Profile facing the primary is the most common option in my experience but I agree some schematics show the schmidt profile facing out.

Originally Posted by rocco

Or doesn't it matter? On my LX200, the corrector plate appears to be flat when I look at the front of the scope, so presumably the 'wave' surface is inside? (I am way too chicken to ever want to pull the corrector plate off to check this!)

It's ok to pull it out. Centering is not critical. Axial orientation is and also orientation of the secondary in its cell.


Meade correctors are much thicker than Celestrons. Celestrons's are matched to the primary. It would be very hard (near to impossible) to find out which surface is curved or not visually. One way to very easily check it though is an optical flat (expensive) or put the corrector on a level turn table then reflect a laser beam at 45 degrees on it. You can ick up which side is the curved one quite easily.

Originally Posted by rocco

How thick is the plate, or does this thickness depend on the aperture of the scope - so would a 14” plate be thicker than an 8” plate?

Yes - I'd say so. Both for optical and mechanical reasons. One thin corrector on a larger aperture scope would be more brittle.

Originally Posted by rocco

And apparently it is perfectly permissible to have a plate that is corrected on both sides and some diagrams reflect this, too.

Yes it seems that way. Maybe some are hand figured after being fitted in.